The present invention relates generally to inverter circuits for converting dc voltage to ac voltage and, more particularly, to a control circuit for generating drive signals for an inverter circuit for converting dc voltage to three phase variable amplitude and frequency ac voltage. While the invention of the present application is generally applicable to such power conversion, it is particularly applicable to a controller for driving adjustable ac motors for example as used in applications such as traction drives for electrically propelled rail vehicles.
Typically in the conversion of dc voltage to ac voltage, an ac or sine wave reference signal is compared with a higher frequency triangular wave to create a pulse width modulation (PWM) signal proportional to the reference signal. The resulting PWM signal is used to drive a power switching inverter which converts dc voltage into ac voltage and is normally constructed of unidirectional conducting switching elements such as power transistors, thyristors, gate turn off (GTO) devices or the like. The PWM mode of operation must ultimately give way to a square wave mode of operation to obtain the maximum possible output voltage from any conversion arrangement. Unfortunately, a problem is encountered in the transition between, triangle interception PWM and unmodulated square wave modes of operation.
A number of attempts have been made to solve this problem. One proposed solution is to provide a series of transitional PWM modes of operation wherein the timing waveform is synchronized to the reference signal, its frequency or slope is variously modified, and/or the amplitude of the reference signal is varied as necessary to reduce the number of "chops" or transitions of the signal which constructs the ac voltage to zero without discontinuity in the amplitude of the fundamental output waveform.
Another proposed solution is disclosed in U.S. Pat. No. 4,047,083 wherein a control arrangement is made up of three modes of operation: the first mode is a triangle interception PWM operating mode which is used as long as an amplitude control signal does not exceed a predetermined reference value and the speed of a controlled motor does not exceed a predetermined reference speed; the second operating mode is a dual dc level set transition mode in which the lower level is varied as a function of the higher level so as to minimize selected harmonics of the ac voltage and the higher level is varied as a function of the amplitude command signal to vary the amplitude of the fundamental voltage component; and the third operating mode is a square wave mode. Transition from the second mode to the third mode is performed by transition means forming a part of the patented invention. For additional information regarding these prior art control arrangements, reference should be made to U.S. Pat. No. 4,047,083 which is incorporated herein by reference.
While the control arrangement of the referenced U.S. patent provides a substantial improvement over prior existing and proposed arrangements, there remains a need for an improved simplified control arrangement for generating drive signals for an inverter circuit for converting dc voltage to three phase variable amplitude and frequency ac voltage which is inexpensive, versatile and adaptable to the requirements of specific applications. Such improved arrangements not only advance the art but also provide attractive alternatives thereto while improving the performance of systems incorporating them.